In a bowl, mix sliced chicken with 2 tbsp soy sauce, 1 tsp black pepper, and cornstarch. Marinate for 10–15 minutes.
- In a small bowl, whisk together 2 tbsp soy sauce, oyster sauce, dark soy sauce (if using), broth or water, brown sugar, and 1 tsp black pepper. Set aside.
- Heat 1 tbsp oil in a large skillet or wok over medium-high heat. Add chicken and stir-fry until golden and nearly cooked through. Remove and set aside.
- Add another tbsp of oil to the pan and sauté mushrooms until browned and tender.
- Add garlic and ginger, cooking 1–2 minutes until fragrant.
- Return chicken to the pan and pour in the prepared sauce. Stir and simmer 2–3 minutes until sauce thickens and coats evenly.
- Drizzle with sesame oil if desired and garnish with green onions. Serve hot with rice or noodles.